What is your turn order?
I am guessing Rex/Chirrut gets off tenacity up before Nihilus uses drain force?
Ideally you want one AOE going off before Nihilus moves (most probably do EP but I think Vader would be a better choice for place lots of debuffs for drain force). Make Vader fast with as little crit chance as possible to hopefully not fill TM against them.
If you can pull that off then Maul's Daze will prevent TM gains from critical hits and hopefully you can take out their cleansers before they can cleanse.